類的構造函數：

類的構造函數是，每當我們創建該類的新對象時執行一類特殊的成員函數。

構造函數都會有完全相同的名字作為類，它冇有任何返回類型可言，甚至冇有作廢。構造可以為某些成員變量設置的初始值非常有用。

下麵的示例說明構造函數的概念：

#include <iostream>
 
using namespace std;
 
class Line
{
   public:
      void setLength( double len );
      double getLength( void );
      Line();  // This is the constructor
 
   private:
      double length;
};
 
// Member functions definitions including constructor
Line::Line(void)
{
    cout << "Object is being created" << endl;
}
 
void Line::setLength( double len )
{
    length = len;
}
 
double Line::getLength( void )
{
    return length;
}
// Main function for the program
int main( )
{
   Line line;
 
   // set line length
   line.setLength(6.0); 
   cout << "Length of line : " << line.getLength() <<endl;
 
   return 0;
}

當上述代碼被編譯和執行時，它產生了以下結果：

Object is being created
Length of line : 6

帶參數的構造函數：

默認的構造函數冇有任何參數，但如果需要，構造函數可以有參數。這有助於在其創建時的初始值賦給一個對象作為顯示，如下麵的例子：

#include <iostream>
 
using namespace std;
 
class Line
{
   public:
      void setLength( double len );
      double getLength( void );
      Line(double len);  // This is the constructor
 
   private:
      double length;
};
 
// Member functions definitions including constructor
Line::Line( double len)
{
    cout << "Object is being created, length = " << len << endl;
    length = len;
}
 
void Line::setLength( double len )
{
    length = len;
}
 
double Line::getLength( void )
{
    return length;
}
// Main function for the program
int main( )
{
   Line line(10.0);
 
   // get initially set length.
   cout << "Length of line : " << line.getLength() <<endl;
   // set line length again
   line.setLength(6.0); 
   cout << "Length of line : " << line.getLength() <<endl;
 
   return 0;
}

當上述代碼被編譯和執行時，它產生了以下結果：

Object is being created, length = 10
Length of line : 10
Length of line : 6

使用初始化列表來初始化字段：

使用初始化列表來初始化字段：

Line::Line( double len): length(len)
{
    cout << "Object is being created, length = " << len << endl;
}

上麵的語法等於如下語法：

Line::Line( double len)
{
    cout << "Object is being created, length = " << len << endl;
    length = len;
}

如果對於一個C類，則具有多個字段的X，Y，Z等，進行初始化，然後用可以使用相同的語法和由逗號分隔字段如下：

C::C( double a, double b, double c): X(a), Y(b), Z(c)
{
  ....
}

類析構函數：

析構函數是執行一個類的特殊成員函數時，它的類的對象超出範圍或當刪除表達式應用到一個指向類的對象。

析構函數將有完全相同的名稱作為前綴類帶有波浪號（〜），它可以冇有返回值，也不能采取任何參數。析構函數可以釋放資源，跳出程序很像在關閉文件之前，釋放內存等非常有用

以下舉例說明析構函數的概念：

#include <iostream>
 
using namespace std;
 
class Line
{
   public:
      void setLength( double len );
      double getLength( void );
      Line();   // This is the constructor declaration
      ~Line();  // This is the destructor: declaration
 
   private:
      double length;
};
 
// Member functions definitions including constructor
Line::Line(void)
{
    cout << "Object is being created" << endl;
}
Line::~Line(void)
{
    cout << "Object is being deleted" << endl;
}
 
void Line::setLength( double len )
{
    length = len;
}
 
double Line::getLength( void )
{
    return length;
}
// Main function for the program
int main( )
{
   Line line;
 
   // set line length
   line.setLength(6.0); 
   cout << "Length of line : " << line.getLength() <<endl;
 
   return 0;
}

讓我們編譯和運行上麵的程序，這將產生以下結果：

Object is being created
Length of line : 6
Object is being deleted